Extend the list of symbols which can be used in custom date format with the X for Time zone offset with a colon.

For parsing a DateTime with a timezone, only the use of Z is mentioned (Z, ZZ, ZZZ, ZZZZ), but this only refers to timezones without a colon.
For parsing a DateTime with a timezone with a colon, the use of X (X, XX, XXX, XXXX) should be added.

Also some explanation would be helpful about when to use Z, ZZ or ZZZ. What is the difference? Is there a difference at all? Same for X, XX or XXX.